The present invention relates to a global positioning system (GPS) receiver apparatus with improved performance and tracking ability in jamming and low signal strength environments.
In military and commercial GPS applications, the GPS signal is susceptible to intentional and/or unintentional reductions in the signal-to-interference-plus-noise ratio (SINR), which could cause the GPS receiver to lose track of the satellite. Reduction in the SINR could be a result of jamming (an increase in the level of interference) or of attenuated signal strength. For military applications, reduction in the SINR typically comes from an intentional jamming source. For commercial applications, unintentional radio frequency (RF) interference or building obstructions can reduce the SINR for a particular satellite.
A GPS receiver initially acquires satellite signals by searching the sky for available satellites. The receiver often has information about the availability and relative location of satellites, based on stored almanac information and/or user input of an approximate current position and time. Since each satellite transmits its own unique pseudo-random noise (PRN) code, the receiver searches for a particular satellite by locally generating the corresponding PRN code sequence, processing the RF signal received by the receiver antenna, and xe2x80x9cmixingxe2x80x9d the PRN code sequence with the RF signal. Once the receiver is able to track the signals from a set of at least four satellites, it transitions to navigation mode.
The satellites transmit their unique PRN code sequence at a particular carrier frequency. As a result, tracking of the satellites requires maintaining an estimate of the signal carrier frequency and/or phase, as well as the PRN code timing. However, the ability to track the carrier frequency is typically lost when the SINR gets too low, although code tracking may still be possible. As the SINR is reduced further, code tracking fails as well.
Many methods have been employed in GPS systems to counteract the effects of jamming, including antenna nulling/beam steering and various algorithmic methods such as extended range correlation (ERC) techniques. ERC techniques enable tracking of the code phase of satellite signals in reduced SINR environments. These techniques have been employed in military GPS receivers to improve their performance in low SINR situations. As SINR drops even further, though, even ERC techniques cannot always maintain code tracking.
Code and carrier tracking of satellite signals in a GPS system is done to provide what are known as pseudorange and deltarange measurements. These measurements are manipulated by a navigation processor to form a position, velocity and time solution. The deltarange measurements are direct observations of velocity, with some error associated therewith. The pseudorange measurements provide observations of position, again with some error associated therewith. A GPS receiver is capable of navigating with only pseudorange measurements, although the velocity solution provided on the basis of pseudorange measurements only suffers from increased noise. The carrier frequency/phase tracking loop of the GPS receiver provides the deltarange measurements and the code phase tracking loop provides the pseudorange measurements.
A typical ERC-enabled GPS system employs a component such as a Kalman filter to use pseudorange measurements to form an estimate of, among other things, position and velocity. The velocity information is used to aid all channels in tracking the carrier frequency. Specifically, the GPS receiver must generate a carrier frequency estimate even when carrier tracking is not possible due to a reduced SINR. Since the satellite being tracked is moving in space with respect to the earth, and since the GPS receiver may be moving relative to the earth, a Doppler effect will cause the apparent carrier frequency manifested by the satellite at the GPS receiver to change. The carrier tracking loop performed by the GPS receiver tracks these changes in carrier frequency, and provides a numerically controlled oscillator (NCO) with appropriate commands to adjust the locally generated estimate of the carrier frequency. The Kalman filter provides each channel with an estimate of the Doppler effects, which is used to help adjust the NCO commands to estimate the carrier frequency when accurate carrier tracking is not possible.
Although ERC techniques as generally described above have improved the low SINR performance of GPS systems, further improvements are desirable for extremely low SINR situations, such as may occur due to jamming in a military application or due to environmental considerations for GPS systems used in a large city, for example. The present invention enables further improvements in GPS system performance by implementing a vector-type ERC system, with cooperation between multiple channels.
The present invention is an apparatus and method for tracking signals transmitted from satellites in a GPS network. A GPS receiver antenna receives the signals transmitted from the satellite, and the signals are converted into digital input signals for each satellite on individual channels. A pseudorange error measurement is generated for each individual channel, and the pseudorange error measurements for all of the individual channels are operated on to generate line-of-sight (LOS) signal tracking commands for each individual channel that are based on the pseudorange error measurements for all of the individual channels. Timing and frequency states for each individual channel are updated based on the LOS signal tracking commands for the respective individual channel.